Combo Floppy Drives

[modem7]

If the BIOS performs double-stepping, then that would mean those programs you mentioned that access the controller directly would not work on a 1.2MB drive (when using 360k disks).

Such a program (just like the code in the BIOS) can establish if a formatted 360K floppy is inserted in a 1.2M drive, and in that case do the double stepping itself.

One other thing I'm curious about. The later XTs were apparently available with internal 3.5" floppies. I've seen pictures of this setup; the drives were black and had a blue eject button. IBM must have used a different floppy controller in those XTs.

The 5160 section of Mueller's Upgrading & Repairing PCs (first edition) includes, "A recent option is a 3.5 inch 720K floppy disk drive. The 3.5 inch drives are available in a normal internal configuration or as an external device." And so being 720K drives, the 'normal' controller could be retained.

 

[Chuck(G)]

Now my Pentium still has it's original 3.5" floppy in it. I'm not using it at the moment, but I found that if you connect it, you can use it alongside one of the combo's two drives (either the 3.5" or 5.25"; you can't use them both at the same time if there's another drive on the cable).

My idea was that you could simply use the combo's 3.5" drive in an XT this way. Of course, for it to work, you'd have to have a second floppy in the XT to prevent it from trying to use the 1.2MB drive.

One other thing I'm curious about. The later XTs were apparently available with internal 3.5" floppies. I've seen pictures of this setup; the drives were black and had a blue eject button. IBM must have used a different floppy controller in those XTs.

As Modem7 has pointed out, the 720K format is nothing more than the 360K format with 40 added cylinders; same data rate. 1.2MB drives, however, are a whole different affair--mostly being descended from 8" drives (360 RPM, 500Mbps data rate). In fact, the NEC 9801 series smoothly progressed through 8" to 5.25" to 3.5" 360 RPM drives without any major changes to software--the capacity is exactly the same on all drives.

Leave it to the USA to do things in the "let a hundred flowers bloom" way with the PC platform.

Back in the 80's, there was a gizmo (I don't recall what it was called offhand) that was an ISA plug-in switch board that allowed you to take a 2-floppy system and support 4 floppies without changing the controller out. Perhaps a manual switch could be rigged for your setup.

 

[Fallo]

The 5160 section of Mueller's Upgrading & Repairing PCs (first edition) includes, "A recent option is a 3.5 inch 720K floppy disk drive. The 3.5 inch drives are available in a normal internal configuration or as an external device." And so being 720K drives, the 'normal' controller could be retained.

Ok, I found out the details. The internal 3.5" drives that were offered in late XTs (and ATs) were made by Alps, and they had edge connectors.

 

[Great Hierophant]

This combo drive is the Epson SD-880, and it is composed of bolting two slimline drives together. The two drives are connected by a thin ribbon cable, which is twisted. There is a jumper on the circuit board to select which drive is A: and B:. You can also do the selection by changing which connector on a two-connector floppy drive cable is fitted to the card edge.

Interestingly, it also has a jumper to set the rotational speed of the 5.25 drive from 360K to 300K.

http://groups.google.com/group/comp...1f10e76?hl=en&ie=UTF-8&q=Epson+SD-880+jumpers

 

[Chuck(G)]

Wish I would have know about those 18 years ago when I had to provide a bunch of combo drives that would handle 1.3MB 5.25" and 3.5" floppies. I did discover a trace that could be jumpered on the Teac 505, but it wasn't easy.